Layered heaters are typically used in applications where space is limited, when heat output needs vary across a surface, where rapid thermal response is desirous, or in ultra-clean applications where moisture or other contaminants can migrate into conventional heaters. A layered heater generally comprises layers of different materials, namely, a dielectric and a resistive material, which are applied to a substrate. The dielectric material is applied first to the substrate and provides electrical isolation between the substrate and the electrically-live resistive material and also reduces current leakage to ground during operation. The resistive material is applied to the dielectric material in a predetermined pattern and provides a resistive heater circuit. The layered heater also includes leads that connect the resistive heater circuit to an electrical power source, which is typically cycled by a temperature controller. The lead-to-resistive circuit interface is also typically protected both mechanically and electrically from extraneous contact by providing strain relief and electrical isolation through a protective layer. Accordingly, layered heaters are highly customizable for a variety of heating applications.
Layered heaters may be “thick” film, “thin” film, or “thermally sprayed,” among others, wherein the primary difference between these types of layered heaters is the method in which the layers are formed. For example, the layers for thick film heaters are typically formed using processes such as screen printing, decal application, or film dispensing heads, among others. The layers for thin film heaters are typically formed using deposition processes such as ion plating, sputtering, chemical vapor deposition (CVD), and physical vapor deposition (PVD), among others. Yet another series of processes distinct from thin and thick film techniques are those known as thermal spraying processes, which may include by way of example flame spraying, plasma spraying, wire arc spraying, and HVOF (High Velocity Oxygen Fuel), among others.
In layered heater applications where the substrate is disposed around or within the part or device to be heated, such as that disclosed in U.S. Pat. No. 5,973,296, which is commonly assigned with the present application and the contents of which are incorporated herein by reference in their entirety, intimate contact between the substrate and the part to be heated is highly desirable in order to improve heat transfer between the layered heater and the part and thus overall heater response. In known layered heaters, however, at least some small air gap is present between the substrate and the part due to inherent fit tolerances, which negatively impacts heat transfer and the response of the layered heater. Other known heaters employ another material on assembly of the substrate to the part, for example, a compound in the form of a thermal transfer paste that is applied between the substrate and the part. During initial operation, however, this compound often produces smoke that could contaminate the heater and/or the surrounding environment. Additionally, application of the compound is time consuming and may also result in some remaining air gaps.
In addition to improved heat transfer as described above, it is often desirable to vary the temperature profile or wattage distribution of electric heaters for certain applications. One known approach to obtain a variable wattage distribution is to vary the width and/or spacing of a resistive circuit pattern within an electric heater. The pattern may be a constant width trace with closer spacing in areas where more heat is desired and wider spacing in areas where less heat is desired. Additionally, the width of the trace may be varied in order to achieve the desired wattage distributions. However, these forms of tailoring the temperature profile or wattage distribution of electric heaters also suffer from reduced, unpredictable, and unrepeatable heat transfer characteristics when undesirable air gaps are present between the heater and the part.